What needs to be done to add a new activity to existing project that works and compiles but has "Project not ready" in the menu. Please see the picture.
Other than that all works, so I don't know what IDE means by "Project not ready"
(Not an exact answer) Observation:
I don't know exactly what happened, but it works now, all I could notice is that gradle had some error from unclean build likely I must have had some code changes resulting in invisible compilation errors. After good build menus available again.
Please post your "observation" if you don't know exact answer.